Leaking of underground water into sewer and drain pipe systems wastefully reduces the cost-effectiveness of these systems and of any waste water treatment plants into which they feed. Thus, municipal authorities have, for quite a few years, specified that the joints between successive lengths of drainage and sewer pipe should be water tight. This requirement has, in turn, sparked a long-standing and still continuing search for ways of joining underground pipe sections together quickly and economically in a way which will provide a water tight joint, notwithstanding the circumferential variations of the pipe sections themselves and the shear forces and angular misalignment resulting from earth settlement and installation conditions and practices.
Attempts to meet this requirement with the familiar bell and spigot type of sewer pipe have involved providing packing between the interior surface of the bell and the free end or spigot of the next adjacent length of pipe which is inserted into the bell. A wide variety of different kinds of packing have been used, including for example: hardenable paste-like preparations; fiber "rope" , e.g. hemp packing; O-rings; and a variety of other types of flexible seals.
Much has been written about the special care and problems involved with bell and spigot pipes and packing. Although some researchers have probed the use of so-called "plain-end" pipes--having walls of uniform diameter throughout their length -- and of joining them by short cylindrical couplings of various configurations, the present inventors know of no widely accepted coupling of this type which is sufficiently shear resistant. The bell and spigot has been for decades -- and still is today -- the dominant factor in the sewage and drainage pipe field.
And the dominance of the bell and spigot type of pipe has continued notwithstanding knowledge in the art that "plain-end" pipe has certain economic advantages, especially in the case of vitrified clay pipe. Because such "plain-end" pipe has no bell, the sections of pipe may be stacked closer together in the dryers and kilns used in manufacturing the pipe. The productivity of the dryers and kilns can be increased, while simultaneously increasing the number of pipe sections which can be processed with a given amount of fuel. Also, pipe sections formed without bells seem to be less prone to cracking during manufacture, so that less scrap is produced. Unfortunately, a completely satisfactory solution to the challenge of satisfactorily coupling "plain-end" pipe sections has defied years of research.
Some of the available types of cylindrical couplings for plain end pipe are costly. Others are not sufficiently durable or effective. Some fail to remain water tight when subjected to substantial shear force and/or axial misalignment. Still others are effective only when the pipe ends are substantially free of circumferential variation, e.g. variations in roundness and circumferential length. The costs which would be involved in manufacturing clay pipe free of such variations has thus far been a prohibitive factor in the adoption of the last-mentioned couplings.
Thus, a need remains for underground sewer or drainage pipe couplings and coupling-pipe combinations which can be quickly and easily joined in the field to form pipe assemblies having stable joints, e.g. joints which can accomodate the usual circumferential variations in clay pipe and yet remain water tight when subjected to substantial shear force and axial misalignment. It is the object of the present invention to meet this long-standing need.